Micro-total analysis systems (μ-TAS or Microfluidic chips) may be used for biological or chemical assays. For example, μ-TAS may be used to perform biological assays using external control lines that control the opening and closing of on-chip fluidic valves which control the flow of fluids in biological assays. The valves are opened and closed using macroscopic pressure sources that are located off-chip, and which are connected through control lines to the chip.
Micro-fluidic valves have been successfully developed using multilayer soft lithography or layering with patterned rigid and elastomeric materials. These methods hold discernible advantages over microelectromechanical (MEMS) valves such as case of fabrication, simplicity of design, and low actuation force requirements. Individual valves of this kind can be compared to an electronic switch, where an outside stimulus is required for control. This technology has been combined and utilized for the fabrication and operation of micro-fluidic on/off valves, switching valves, and pumps.
A limitation of these types of approaches is that, because each individual valve is analogous to an electronic switch, each valve requires a separate pressure (positive or negative) control line. This type of component can be classified as an active micro-fluidic component. Multiple valves, pumps, etc., are desirable for most applications; in some cases, a large number of these active components are needed. In these cases, the instrumentation needed for control of these miniaturized devices becomes overwhelming with respect to complexity, cost, and space requirements. Especially inn complex assays, a large number of macroscopic control lines are cumbersome and undesirable.
It is desirable to provide passive micro-fluidic components that allow defined flow control at the small volume scale (microliter, nanoliter, picoliter, or smaller) and are easy to fabricate. Fluidic components analogous to electrical resistors, diodes, inductors, and capacitors could provide this control without the necessity for control lines. Micro-fluidic valves could be used only when absolutely necessary, and the controlling instrumentation could be miniaturized to a scale comparable or more fitting to the microchip scale and platform.
Passive components with diode-like behavior have been developed previously (Holtz et al., Anal. Chem. 1998, 70 (4):780-791; Adams et al., J. Micromech. Microeng. 2005, 15:1517-1521), but these components require multilayer fluid flow and more complex patterning than is desired. Nonetheless, with these types of components, the development of ‘smart’ devices becomes a possibility, in which the fluid control features are built entirely into the devices and not the instrumentation.
Therefore, there remains a need for passively controlling fluid flow in a μ-TAS, without requiring complex control instrumentation or control lines, that can be manufactured inexpensively and easily.